Various touch input devices are known. They typically detect the location on the input surface which is touched and the level of pressure applied to the input surface.
For example, the touch input device described in Japanese Patent Application Laid-Open No. 5-61592 includes a touch sensor obtained by overlapping a flat pressure sensor and a flat position sensor. The pressure sensor detects a distortion in a thickness direction, and therefor detects the level of pressing force applied to the input surface. The capacitive sensor detects the position at which the user touches the input surface.
In Japanese Patent Application Laid-Open No. 5-61592 electrodes are formed on the entirety of opposed surfaces of a piezoelectric substrate to increase an output of the pressure sensor. However, according to this method, the relationship between the amount that the pressure sensor is distorted and the force with which the user presses down on the input surface differs depending on the position (a pressing position) on the input surface against which the user presses his or her finger. More specifically, the amount that the pressure sensor is distorted (for a given pressing force) is greater in the center region of the pressure sensor than it is at the outer peripheral regions of the pressure sensor. That is, even though the pressing forces are the same, the amount of electric charges produced by the pressure sensor varies as a function of the pressing position.
Hence, the touch input device described in Japanese Patent Application Laid-Open No. 5-61592 has a problem that it is necessary to correct an output of the pressure sensor as a function of the location of the pressing position. As a result, the required processing becomes complex.
It is therefore an object of the present invention to provide a touch sensor which can prevent (or at least mitigate) an output from varying as a function of the pressing position.